PROJECT SUMMARY To survive in unique environments animals have evolved a host of mechanisms to maximize the intake, storage, and use of energy. The teleost fish Astyanax mexicanus is a proven model for understanding the genetic basis of adaptation and represents a particularly strong system to investigate evolutionary changes in metabolism. It exists as a river-dwelling form and multiple independently derived eyeless cave-dwelling forms that thrive in perpetual darkness with a limited and infrequent food supply. The river-dwelling and cave-dwelling forms are completely interfertile and can be easily bred in the lab to identify quantitative trait loci for numerous distinct morphological, behavioral, and physiological traits. There is little known about how metabolism has evolved in low nutrient environments to prevent death during long periods or starvation. The enteric nervous system (ENS) is central to regulating metabolism as it orchestrates gastrointestinal (GI) motility, nutrient absorption, and gastric and pancreatic secretion. We find evidence that ENS development is altered in cavefish in a way that could drive differences in intestinal absorption and glucose homeostasis. We have identified cavefish-specific mutations in genes critical for ENS development in humans (EDNRB, EDN3), and have observed that cavefish have fewer enteric neurons in the large intestine, more frequent stomach churning contractions, and less frequent peristaltic wave contractions. These changes may lead to superior intestinal absorption as we find that anti-oxidant carotenoids obtained from food accumulate in the visceral adipose tissue of cavefish, but not river fish. The function of the ENS is also linked to glucose homeostasis and GI disorders frequently accompany diabetes. Interestingly, we find that cavefish have higher blood glucose levels and slower glucose clearance compared to river fish. To understand how these physiological differences may provide an adaptive advantage in a low nutrient environment, we propose the following aims: 1) Determine the contribution of EDNRB and EDN3 alleles to enteric neuron number, GI motility, and bowel transit, 2) To test the hypothesis that accumulation of carotenoids in cavefish visceral adipose tissue is due to superior intestinal absorption, and 3) Investigate the role of insulin signaling in cavefish glucose homeostasis. The principles that emerge from our work will lead to a better understanding of metabolic variation in vertebrate species. Furthermore, our findings may have relevance to human health: mutations in EDNRB and EDN3 are associated with aganglionic megacolon in humans, and there is little known about how human variation in carotenoid bioavailability and glucose homeostasis are linked to ENS function.